As the demand increases for the processing capacity of computers, expansion in the data transmission/reception band between a CPU and a memory or between a number of CPUs has been required. Since data transmission by means of electrical signals is reaching its limitations, the application of optical signals has been required.
At this time, it is necessary to integrate a CPU and light processing components in order to efficiently convert electrical signals within the CPU and the memories to optical signals used for data transmission/reception or vice versa. In response to this issue, research and development in the field that is referred to as silicon photonics (Si-photonics), where various types of optical components are formed on a Si substrate, has been attracting attention.
The part for carrying out a process of optical multiplexing/demultiplexing as well as modulation from among the optical components is required to have such properties as not to absorb light in order to avoid excess loss. Meanwhile, light receiving units (PDs: photodetectors) for converting light to electricity naturally need to have such properties as to absorb light.
What is considered to be a prospective candidate that satisfies this demand is a combination of a Ge material for the light receiving units, an Si material for the remaining portions, and near-infrared rays having a wavelength of 1.2 μm to 1.6 μm that are used as signal light. The Si is transparent for the light of this wavelength range, which is absorbed by the Ge material. Though there are other semiconductor materials that absorb the light of this wavelength range, Ge is a group IV material like Si and has a smaller contamination effect during the manufacturing process as compared to a case where a group III-V compound semiconductor mixed crystal is used, and thus is regarded as the prospective candidate.
Irrelevant of what is used as the material, conventional PDs need a mechanism for taking out electrons and holes that have been generated as a result of light absorption, that is to say, photocarriers, to the outside. Such a mechanism requires an electrode for applying an electrical field to the semiconductor and a band structure distribution for achieving rectification with which electrons and holes are separately taken out. Typically, two types are known as such a mechanism, and they are respectively referred to as an MSM (metal-semiconductor-metal) type PD and a pin type PD.
In the case of an MSM type PD, the metal that forms an electrode has such a structure as to be joined to a semiconductor through a Schottky type interface. This is different from the pin type PD, thereby making it unnecessary to dope the semiconductor portion to which the metal is joined with high concentration impurities, and rectification is achieved by the Schottky barrier in the interface of the junction.
In contrast, in a pin type PD, a band inclination provides rectification in the junction between a p type semiconductor and an n type semiconductor that are formed by injecting high concentration impurities into a semiconductor. In addition, the pin type PD is formed by doping impurities of a particularly high concentration such as approximately 1.0×1020 cm−3 in order to provide an ohmic contact, which does not have rectification in the interfaces between the metal and the semiconductor.
The respective structures have their own advantages and disadvantages. The MSM type structure does not need doping, which makes the manufacturing process simple, and has such characteristics that high speed operation is excellent. Meanwhile, the region that has rectification, that is to say, the region in which photocarriers are to be generated through light absorption, is close to a metal, and therefore, it is difficult to avoid light absorption by the metal, which makes it difficult to provide good properties in terms of the sensitivity of light reception. Another problem arises such that the sensitivity deteriorates due to a recombination of electron and hole pairstrapped by the surface state. In addition, the band structure in the Schottky interface in the junction between a metal and a semiconductor is determined by the work function of the metal and the Fermi level of the semiconductor, and therefore, the properties of the PDs are almost uniquely determined by the materials used therein and have such defects that the controllability is low. In the case of a Ge material that is used in the field of Si-photonics in particular, the Schottky barrier vis-à-vis a typical electrode metal such as Al is low, and therefore, there is a defect that a dark current becomes high, making it easy for the noise properties to deteriorate.
In contrast, the pin type structure requires slightly more complicated steps in the manufacturing process because it is necessary to control the doping profile, and an increase in the area of the pin junction directly leads to an increase in the capacitance of the device, and therefore, the pin type structure has such defects that high speed operation is slightly more difficult. However, the pin type structure has such an advantage that the properties can be adjusted in accordance with the application by appropriately designing the doping profile even when the same materials are used. Furthermore, light can be made to efficiently enter into a very narrow range by using a waveguide type PD, and therefore, it is easy to achieve high sensitivity even when the PD is miniaturized so as to make the capacitance of the device small. Accordingly, it is possible to solve the problems with the high speed operation, which are the above-described defects of the pin type structure. Judging from the above-described comparison between the advantages and the disadvantages, a waveguide type Ge-pin type light receiving device has been diligently researched and developed as the most prospective structure in the current Si-photonics field (see Patent Literature 1 and Non-Patent Literature 1).
FIGS. 18A and 18B are diagrams illustrating a conventional waveguide type Ge-pin type light receiving device. FIG. 18A is a perspective diagram, and FIG. 18B is a cross-sectional diagram along the single-dotted chain line parallelogram in FIG. 18A. A Si fine line waveguide 63, a tapered waveguide 64 and a Si pedestal are formed by processing an SOI (Si on insulator) layer provided on the Si substrate 61 with a BOX (buried oxide) layer 62 in between. This Si pedestal is doped with B (boron) so as to become a p type Si layer 65.
After providing an SiO2 film 66 on the entirety of the surface, an opening is created in the SiO2 film 66, and an i type Ge layer 67 is selectively grown within this opening. The upper portion of the i type Ge layer 67 is doped with P (phosphorous) so as to provide an n type Ge layer 68. A pin type photodiode is formed of the p type Si layer 65, the i type Ge layer 67 and the n type Ge layer 68. Next, an SiO2 film 69 is provided, and after that, contact holes that reach the p type Si layer 65 and the n type Ge layer 68 are created, and these contact holes are filled in with a conductive member so as to provide an n side electrode 70 and a p side electrode 71. In this case, the BOX layer 62 works as a lower clad layer, and the SiO2 films 66 and 69 work as an upper clad layer, and therefore, light is confined within the Si fine line waveguide 63.
Ge has a refractive index that is higher than that of Si, and therefore, light that propagates through the Si fine line waveguide 63 is led to the i type Ge layer 67 through evanescent optical coupling and absorbed so as to generate photocarriers. A reverse bias is applied between the p side electrode 70 and the n side electrode 71 so that the n type Ge layer 68 is at a higher voltage relative to the p type Si layer 65 so that the photocarriers are drawn out, which makes the system work as a photodiode.